Piezoelectric transformers utilizing the piezoelectric effect are known in the art. A piezoelectric transformer can be defined as a passive electrical energy-transfer device or transducer employing the piezoelectric properties of a material to achieve the transformation of voltage or current or impedance. Piezoelectric transformers have recently replaced wound-type electromagnetic transformers for generating high voltage in the power circuits for certain electronic applications. These piezoelectric transformers offer numerous advantages over ordinary electromagnetic transformers including a compact and slim shape, rugged construction, and high reliability in a comparably smaller package.
Piezoelectric ceramic transformers are now finding applications in a variety of applications including photocopiers, backlights of liquid crystal displays (LCDs), flat panel displays (FPDs), power converters, CRT displays, field emission displays (FEDs), and the like. FIG. 1 shows the construction of a Rosen-type piezoelectric transformer, a representative example of a piezoelectric transformer of the prior art.
FIG. 1 shows a standard multilayer ceramic piezoelectric transformer 100 formed from a co-fired ceramic package 102 interspersed with layers of a conductive metallization electrode pattern 104. Although the package is made from layers of a green ceramic tape, upon firing the package sinters into a single ceramic structure. Consequently, all FIGS. will show the fired package and the individual layers of green tape will not be shown. A voltage (Vin) is applied across the electrode layers 104 creating a potential difference across the primary region 106. The primary region 106 is polarized in the direction of its thickness, as indicated by the arrows between the electrode layers 104 shown in FIG. 1. Polarization is a process wherein a very substantial direct current (DC) voltage, in the range of 4 kV/mm, is applied to the ceramic in order to give the material its piezoelectric properties. Similarly, a portion of the piezoelectric transformer 100 indicated by reference numeral 108 is a power generating or secondary section. An output (Vout) is formed on the end face 110 of transformer 100 corresponding to the power generating section 108. Note that the end face 110 is metallized with a conductive coating, as is the top surface 112 on the primary section 106 end of transformer 100. Electrode layers 104, internal to the multilayer package, are also made from a conductive coating material. The power generating section 108 is polarized in the lengthwise direction, as indicated by an arrow in FIG. 1.
With reference to FIG. 1, the basic operation of a piezoelectric transformer can be understood. When a voltage is applied to the primary section 106 of the piezoelectric transformer, the resulting electric field causes a vertical vibration due to a change in the thickness of the driving section 106 of the transformer. This vertical vibration results in a horizontal vibration in the lengthwise direction (also known as the piezoelectric transverse effect 31 mode) along the entire length of the transformer 100. In the power generating section 108 (also known as the secondary section), a voltage having the same frequency as that of the input voltage (Vin) is derived through the output electrode (Vout) in accordance with a piezoelectric effect wherein a potential difference occurs in the polarizing direction due to the mechanical strain in the polarizing direction. At this time, if the driving frequency is set to be the same as the resonant frequency of the piezoelectric transformer, a very high output voltage can be obtained. Stated another way, by applying an alternating current through the primary section 106 of a piezoelectric material, its thickness characteristics will vary, which through coupling, causes a change in length of the secondary section 108, which results in a change in the electrical output caused by an electromechanical effect.
FIG. 2 shows another prior art piezoelectric transformer in which the primary 206 and the secondary 208 sections of the piezoelectric ceramic plates are "stacked" vertically. Referring to FIG. 2, a stack-type transformer 200 is provided. Once again, a voltage (Vin) is applied to a primary 206 section through a series of metallized electrode layers 204, which in turn results in a high output (Vout) in the secondary 208 section due to a piezoelectric effect. Top surface 210 is covered with a conductive coating, and side surfaces 212 and bottom surface 214 are selectively metallized to achieve the desired transformer properties. In stacked transformer 200, all layers are polarized in the thickness direction as is designated by the arrows shown in FIG. 2.
One problem with the present technology is that a single piece of electronic equipment may contain multiple transformers, each with its own drive circuit, and each with its own output voltage. Thus, by providing multiple outputs in a single transformer design, a substantial reduction in both size and weight may be realized. Additionally, greater integration is possible in a multiple output transformer having a single drive circuit with a small footprint. Finally, by decreasing the overall number of transformers in a single piece of electronic equipment, a cost savings is also obtained. Another problem with these transformer designs, and all other piezoelectric transformers in the field, is the fact that they have only a single driven section (power generating section) resulting in only a single output voltage. However, system designers for various applications are demanding multiple output voltage capabilities. A novel ceramic piezoelectric transformer structure which offers multiple driven sections resulting in multiple outputs wherein a single a input voltage drives two or more output voltages would be considered an improvement in the art.